1. Field of the Invention
The present invention relates generally to communication circuits and more particularly to a circuit for transmitting a high-frequency communication signal over a regular a.c. power line for line load control.
2. Prior Art
Power line communication circuits are used whenever it is desired to remotely control the operation of a load distributed on the ac. power line at various locations whereby the load may be a house lighting system, alarm system or the like. The prior art has numerous references to relatively complicated and largely inefficient communication circuits which perform various remote control functions over a regular a.c. power line. For example, U.S. Pat. No. 4,024,528 to Boggs et al teaches a remote switching system in which operation of a load is controlled by signals generated at a remote location and transmitted along an a.c. power line. The circuit includes a signal transmitter which is powered by the a.c. power line source and which operates to place a transient control signal on the power line. Further included is a receiver which is electrically connected to the line load for remote control of the same. The control signals are applied to the line during an early time in a half-cycle of the power line voltage before noise transient signals generally appear on the half-cycle. This type of transmission control system has a rather complicated circuit architecture which contributes to an inefficient operation.
Other power line communication systems utilize capacitor or transformer couplings to couple the carrier signal to the power line and expensive push-pull amplifiers to actively supply and draw current to/from the power line. Such power line communication systems also require relatively expensive and powerful power supplies, have inefficient operation and may not be easy to implement in a typical house or office setting.
Therefore, the need arises for a new power line communication circuit which can operate efficiently from a relatively compact and inexpensive power supply which needs to produce, for example, not more than 100 mA. Such a communication circuit should have a simplified circuit architecture in which preferably the coupling of the communication signals to the a.c. power line can be accomplished without the use of capacitors, transformers or the like.
The present invention is directed to a circuit for transmitting communication signals over an alternating current (a.c.) low-frequency power line, the circuit comprising a rectifier operatively coupled to the a.c. power line for generating an unfiltered rectified direct current (d.c.) voltage signal; and a voltage-to-current converter (VCC) operatively coupled to the rectifier for receiving the unfiltered rectified d.c. voltage signal, the VCC adapted for receiving a high-frequency a.c.-shaped communication voltage signal and for generating an output current in response to the high-frequency a.c.-shaped communication voltage signal and the unfiltered rectified d.c. voltage signal, the output current including a high-frequency a.c. component from the high-frequency a.c.-shaped communication voltage signal superimposed on a low-frequency d.c. component from the unfiltered rectified d.c. voltage signal, the superimposed high-frequency a.c.-shaped signal component placing communication signals on the a.c. power line through the rectifier.
In accordance with one aspect of the present invention, the circuit further comprises a receiver operatively coupled between the rectifier and the VCC for providing a closed loop high-frequency a.c.-shaped communication voltage feedback signal, the high-frequency a.c.-shaped communication voltage feedback signal summed with the high-frequency a.c.-shaped communication voltage signal at a summing junction to control the size and shape of the high-frequency a.c.-shaped communication signal applied to the VCC.
In accordance with another aspect of the present invention, the rectifier comprises a half-wave rectifier. The half-wave rectifier includes a rectifier diode, the rectifier diode having a cathode and an anode, the anode operatively coupled to the a.c. power line for drawing current from the a.c. power line during the positive half cycle of the a.c. power line.
In accordance with yet another aspect of the present invention, the VCC comprises an N-channel field effect transistor (FET) having a source, a drain and a gate, the drain connected in series with the cathode of the rectifier diode for receiving the unfiltered rectified d.c. voltage signal during a positive half cycle of the a.c. power line, and an integrator having a non-inverting input terminal operatively coupled to the summing junction for receiving the summed high-frequency a.c.-shaped communication voltage signal, an inverting input terminal biased by a biasing voltage source, and an output terminal operatively coupled to the gate of the FET, the integrator generating a gate control voltage in response to the high-frequency a.c.-shaped communication voltage signal and the biasing voltage, the gate control voltage applied to the gate of the FET for modulating the output current from the FET, the modulated output current including a high-frequency a.c. component from the high-frequency a.c.-shaped communication voltage signal superimposed on a low-frequency d.c. component from the unfiltered rectified d.c. voltage signal, the superimposed high-frequency a.c.-shaped signal component placing communication signals on the a.c. power line. The VCC further comprises a feedback network operatively coupled between the integrator and a current sensing resistor for stabilizing the voltage output from the integrator, the current sensing resistor operatively coupled to the source of the FET, the feedback network comprising a voltage divider operatively coupled to the current sensing resistor. The feedback network further comprises a lead-lag network including an RC filter coupled in parallel with a resistor, the resistor being part of the voltage divider.
In accordance with still another aspect of the present invention, the receiver includes a first RC filter operatively coupled to the cathode of the rectifier diode for receiving the output current and decoupling the low-frequency d.c. component from the high-frequency a.c.-shaped communication signal component, a second RC filter operatively coupled to the first RC filter for filtering the decoupled high-frequency a.c.-shaped communication signal component, a non-inverting amplifier operatively coupled to the second RC filter for amplifying the filtered high-frequency a.c.-shaped communication signal component, and a feedback resistor operatively coupled between the non-inverting amplifier and the summing junction for closing the feedback loop on the high-frequency a.c.-shaped feedback communication voltage signal.
In accordance with a further aspect of the present invention, the rectifier comprises a full-wave rectifier. The full-wave rectifier includes a four-diode bridge rectifier having a first and second input terminals operatively coupled on the a.c. power line for continuously drawing current from the a.c. power line, a positive output terminal for providing an unfiltered rectified direct current (d.c.) voltage signal, and a negative output terminal, the negative output terminal serving as circuit ground for the circuit.
In accordance with a still further aspect of the present invention, the VCC comprises an N-channel field effect transistor (FET) having a source, a drain and a gate, the drain connected in series with the positive output terminal of the four-diode bridge rectifier for continuously receiving the unfiltered rectified d.c. voltage signal, and an integrator having a non-inverting input terminal operatively coupled to the summing junction for receiving the summed high-frequency a.c.-shaped communication voltage signal, an inverting input terminal biased by a biasing voltage source, and an output terminal operatively coupled to the gate of the FET, the integrator generating a gate control voltage in response to the high-frequency a.c.-shaped communication voltage signal and the biasing voltage, the gate control voltage applied to the gate of the FET for modulating the output current from the FET, the modulated output current including a high-frequency a.c. component from the high-frequency a.c.-shaped communication voltage signal superimposed on a low-frequency d.c. component from the unfiltered rectified d.c. voltage signal, the superimposed high-frequency a.c.-shaped signal component placing communication signals on the a.c. power line through the four-diode bridge rectifier. The VCC further comprises a feedback network operatively coupled between the integrator and a current sensing resistor for stabilizing the voltage output from the integrator, the current sensing resistor operatively coupled to the source of the FET, the feedback network comprising a voltage divider operatively coupled to the current sensing resistor. The feedback network further comprises a lead-lag network including an RC filter coupled in parallel with a resistor, the resistor being part of the voltage divider.
In accordance with a different aspect of the present invention, the receiver includes a first RC filter operatively coupled to the positive output terminal of the four-diode bridge rectifier for receiving the output current and decoupling the low-frequency d.c. component from the high-frequency a.c.-shaped communication signal component, a second RC filter operatively coupled to the first RC filter for filtering the decoupled high-frequency a.c.-shaped communication signal component, a non-inverting amplifier operatively coupled to the second RC filter for amplifying the filtered high-frequency a.c.-shaped communication signal component, and a feedback resistor operatively coupled between the non-inverting amplifier and the summing junction for closing the feedback loop on the high-frequency a.c.-shaped feedback communication voltage signal.
These and other aspects of the present invention will become apparent from a review of the accompanying drawings and the following detailed description of the preferred embodiments of the present invention.